1. Field of the Invention
The present invention relates to a decompression apparatus provided with an exhaust mechanism for decompression, and more particularly, to a decompression apparatus, such as a CVD apparatus, used in a manufacturing process for semiconductor devices.
2. Description of the Related Art
Conventional decompression apparatuses furnished with an exhaust mechanism for decompression include, for example, a processing apparatus used in a thermal diffusion process and filming process for semiconductor devices. This processing apparatus executes processes such as a filming process for an object of processing, e.g., a semiconductor wafer, in a processing unit in a manner such that the internal pressure of the processing unit is reduced to a vacuum pressure by the exhaust mechanism. If the interior the processing unit is set in a vacuum in this manner, impurities, such as oxygen and water in the atmosphere, can be prevented from adhering to the object of processing. In order to get a vacuum in the processing unit, the exhaust side of the processing unit is connected to a vacuum pump by means of an exhaust line. The exhaust line is fitted with a trap device which traps impurities and products from the processing unit by condensing them.
FIG. 3 shows the construction of a processing unit and an exhaust system of a CVD apparatus as an example of the decompression apparatus. This CVD apparatus is an apparatus for forming a thin film on the surface of a semiconductor wafer W by chemical vapor deposition in a semiconductor device manufacturing process. In FIG. 3, numeral 110 denotes the processing unit for a filming process in which the thin film is formed on the surface of the semiconductor wafer W as an object of processing, and numeral 102 denotes the exhaust system for setting the interior of the processing unit 110 in a vacuum. A heater 130 is arranged around the unit 110. A wafer boat 132, which carries thereon a plurality of semiconductor wafers W arranged at regular intervals, is delivered into or from the processing unit 110 by means of a boat elevator 134. The exhaust system 102 comprises an exhaust pump 112 and an exhaust line 114 which connects the pump 112 to an exhaust port of the unit 110. The line 114 is fitted with a trap device 116 and a main valve 118 which are arranged successively along the exhaust direction from the side of the unit 110. When the valve 118 is opened and the exhaust pump 112 is actuated, in this arrangement, the processing unit 110 is exhausted to be evacuated, and impurities and a product from the unit 110 are trapped by means of the trap device 116.
When the main valve 118 is opened in an instant when the processing unit 110 is exhausted, the pressure in the unit 110 is reduced suddenly, so that the semiconductor wafers W in the unit 110 may possibly move, or particles may be flung up in the unit 110. To avoid this, therefore, a bypass line 122 for detouring the main valve 118 is provided in the middle of the exhaust line 114. In the initial stage of exhaust operation, slow exhaust is effected such that gas is discharged gradually or little by little from the processing unit 110 through the line 122. In order to carry out this slow exhaust, the bypass line 122 is fitted with a sub-valve 124 for opening and closing it, and the line 122 is formed having a flow area narrower than that of the exhaust line 114, for example. Thus, if the sub-valve 124 is opened earlier than the main valve 118 so that the processing unit 110 is exhausted gradually through the bypass line 122, in the initial stage of the exhaust operation, the pressure in the unit 110 can be prevented from being reduced suddenly.
The sub-valve 124, which serves to prevent a transient change of pressure in the processing unit 110 in the initial stage of the exhaust operation, is closed when the movement of the semiconductor wafers W and the flinging of the particles cease with the progress of the exhaust operation. Thereafter, the main valve 118 is opened, and main exhaust from the processing unit 110 is effected through the exhaust line 114. Alternatively, the main exhaust from the unit 110 may be effected through both the bypass line 122 and the exhaust line 114 with the line 122 kept open without closing the sub-valve 124.
The main valve 118 is open during the execution of the processes, such as the filming process in the processing unit 110, and so is the sub-valve 124 in some cases. During the processing operation, the product and gas are discharged from the unit 110 into the trap device 116, and are condensed thereby. In order to prevent the product from adhering to the valve itself, in this case, the main valve 118 is provided with heating means for heating it, for example.
The main valve 118 and sub-valve 124 are closed when the semiconductor wafers W are delivered into or from the processing unit 110. In other words, the unit 110 is opened to allow the processed wafers W to be delivered therefrom after the valve 118 and sub-valve 124 are closed so that the internal pressure of the unit 110 is increased from the vacuum pressure to the normal pressure. Also, the semiconductor wafers w are delivered into the processing unit 110 in the normal-pressure state with the main valve 118 and sub-valve 124 closed. In order to increase the internal pressure of the unit 110 from the vacuum pressure to the normal pressure, a process gas introduced into the unit 110 through a process gas inlet pipe 140 is first exhausted. When this exhaust operation is finished, the main valve 118 is closed (and so is the sub-valve 124 if open), and a purge gas, such as nitrogen gas, is introduced into the unit 110.
Increasing the internal pressure of the processing unit 110 from the vacuum pressure to the normal pressure, however, involves the following problems. The trap device 116 and the unit 110 communicate with each other at all times. If the internal pressure of the processing unit 110 is increased from the vacuum pressure to the normal pressure, therefore, a substance adhering to the trap device 116 easily evaporates, diffuses from the device 116 into the exhaust line 114, and flows backward into the unit 110. In a filming process using silicon nitride as the process gas, in particular, ammonium chloride gas is formed as a product. Since the ammonium chloride gas absorbs water, it can evaporate relatively easily, and its evaporation increases as the normal pressure is approached. Accordingly, the ammonium chloride gas, caused to adhere to the trap device 116 during the exhaust operation, is liable to flow backward from the device 116 to the processing unit 110 as the pressure is switched to the normal pressure. This backward flow is also produced in silicon dioxide which is formed in a filming process using TEOS. In some cases, the gas fed back from the trap device 110 into the processing unit 110 in this manner adheres to the semiconductor wafers W delivered into or from the unit 110, thereby soiling the wafers.
In the case where the process gas used forms a product which contains noxious substances, such as phosphorus (BPSG), arsenic, etc., these substances are trapped in the trap device 116. If the processing unit 110 is opened during the delivery of the semiconductor wafers W, therefore, the harmful substances may possibly leak out from the unit 110. If the returned product remains in the unit 110, moreover, setting the interior of the unit 110 under a predetermined vacuum pressure takes a long time in the next cycle of exhaust operation. Accordingly, the standby time before the start of the next filming process is too long to ensure a satisfactory throughput in the semiconductor wafer manufacturing process.